Cryogenic near-field photocurrent studies of gapped and twisted bilayer graphene systems

Berry curvature is analogous to magnetic field but in momentum space and is commonly present in materials with non-trivial quantum geometry. It endows Bloch electrons with transverse anomalous velocities to produce Hall-like currents even in the absence of a magnetic field.  We present various optoelectronic probing schemes of the Berry curvature in gapped and twisted bilayer graphene, employing infrared and Terahertz radiation. One tangible example is the direct observation of in situ tunable valley-selective Hall effect (VSHE), where inversion symmetry, and thus the geometric phase of electrons, is controllable by an out-of-plane electric field.

 

To take this physics further to the nanoscale, we implemented cryogenic (10K) near-field optical microscopy and near-field photocurrent mapping. This technique allows for 20-nm resolution spatial mapping of the local optical properties, collective excitations and Seebeck coefficient. Our observations allow us to relate the spatial variations of correlated electronic states to spatial variations of the twist angle. Moreover, we observe spatial anisotropies, which shed light on the microscopic mechanisms of symmetry breaking.